The present invention generally relates to MEMS (micro-electro-mechanical system) devices. More particularly, the invention relates to a capacitive pressure sensor capable of exhibiting high sensitivity in a small die size, due in part to a pressure-sensing diaphragm mechanically coupled but electrically insulated from a mechanical capacitor that generates the electrical output of the sensor.
As well known in the art, capacitive pressure sensors employ a diaphragm that deflects in response to pressure, so that an electrode carried or defined by the diaphragm moves relative to a fixed (stationary) electrode with which the diaphragm defines a mechanical capacitor. Capacitive pressure sensors can be configured for measuring absolute, differential, and gage pressures, and have the advantage of low power consumption requirements and reduced susceptibility to temperature effects for sensitivity and offset. However, capacitive pressure sensors are susceptible to parasitic capacitances and therefore need to be integrated with interface circuitry. In addition, their dynamic range is limited unless closed-loop operation is utilized and high pressure resolution can be achieved with the sensor structure. High sensitivity in pressure sensors is typically achieved by increasing the diaphragm size, reducing the diaphragm thickness, or some other measure whose effect is to maximize the deflection for a given input pressure. However, increased sensor size is often undesirable or not practical and increased deflection is associated with nonlinearity in the output that requires additional compensation circuitry, with the further disadvantage of consuming more power. Increased size and reduced thickness also render the diaphragm more susceptible to breakage.
Capacitive pressure sensors are also sensitive to their operating environments, including fluidic, chemical, and electromagnetic effects. In some cases the diaphragm, which is typically formed from one or more conductive layers, must be protected from its operating environment while at the same time being capable of deflection in response to changes in pressure within the environment. To promote their media compatibility, diaphragms can be protected with, for example, an elastomeric layer or a incompressible fluid such as an oil that transmits environmental pressure to the diaphragm. Protection of the sensor and interface circuit from electromagnetic effects is generally accomplished at the package or module level by providing an electromagnetic interference (EMI) shielding layer, which can be complex and expensive to implement. The integration of a pressure sensor with CMOS interface circuitry can also be challenging because of restrictions imposed by the CMOS fabrication process due to thermal budget and topography.
In view of the foregoing, a need exists for capacitive pressure sensors that are capable of being integrated with a CMOS interface circuitry and exhibiting high sensitivity and a wide dynamic range within a small die size, while also being immune to adverse media and electromagnetic effects.